Dispersity

Dispersity, in polymer science and colloid chemistry, refers to the breadth of a distribution of sizes or masses within a sample. In polymers, it is quantified by the dispersity index Đ, defined as the weight-average molecular weight Mw divided by the number-average molecular weight Mn (Đ = Mw/Mn). An ideal monodisperse sample has Đ = 1, meaning all molecules have the same molecular weight; real systems typically have Đ > 1. Living or controlled polymerization methods can approach Đ close to 1, while conventional free-r radical or step-growth processes often yield broader distributions.

Measurement and interpretation: Đ reflects distribution breadth, not the exact shape. Commonly, polymer molecular weights are measured by gel permeation chromatography (GPC/SEC), which provides Mw and Mn relative to calibration standards. Absolute Mw can be obtained by methods such as multi-angle light scattering (MALS). It is important to note that different distributions can yield the same Đ, and Đ does not specify distribution form.

Implications: The dispersity influences physical properties such as viscosity, crystallinity, mechanical strength, and thermal behavior, as well as processing characteristics like film formation and melt behavior. A narrow dispersity (Đ close to 1) enables more uniform properties, while broader dispersity can broaden transitions and complicate processing, though in some cases it may aid certain applications or processing steps.

Other contexts: The term dispersity also appears in colloid science to describe particle-size distributions. The polydispersity index (PDI) is often used interchangeably with Đ in polymer contexts. See also molecular weight distribution.